The use of a number of bent rods as a bending mold is known where each of the rods is rotatable about an axis passing through its two ends and where each rod is covered by a tubular casing which is driven in rotation. By causing a sheet of glass heated to the plastic state to rest on a flat bed made up of such rods lying in the same longitudinal plane, it is possible, as described in U.S. Pat. No. 3,545,951, to impart a bending to the sheet by causing all the rods to simultaneously pivot to a common angle. Thus, when the bed formed by the bent rods ceases to be flat, the sheet of glass in the plastic state takes the approximately cylindrical shape imparted by all the raised, bent rods, either by sinking at its edges under its own weight or by sinking under the pressure of a counterform. It is possible to obtain sheets which are more or less bent in this manner, with the bending being nil when the rods all lie flat in one plane, the bending amounting to a maximum when all the rods are pivoted through a right angle with respect to the plane, and the bending being intermediate between those two extremes when the rods assume an intermediate inclination with respect to the plane.
In accordance with U.S. Pat. No. 4,054,438, it is also known that bending can be accomplished in a progressive manner by utilizing a shaping bed having progressive convexity with the bed being made up of similar shaped bent rods. The first rod in such a bed lies flat in the plane of the rollers which move the sheet to the bending station. The first rod is followed by intermediate rods which are fixed in progressively raised positions approaching the fixed position of the last rod in the shaping bed with the last rod imparting the desired degree of bending to the sheet. Thus, the sheet of glass is bent as it moves along, without any need to change the position of the rods, which are adjusted once and for all for production of sheets of a given radius of curvature.
In such installations, and particularly in the installation disclosed in the U.S. Pat. No. 4,054,438, the adjustments to change the radius of curvature are very easy and can be made rapidly. However the sheets of glass are only carried by bent rods which are spaced relatively far apart. Relatively large spacing between the rods is required in order to accommodate the means for varying the inclination of the rods by which the radius of curvature of the mold formed by the rods is changed. The result is that the sheets of glass acquire undulations which are perpendicular to their axes of bending and which consequently can interfere with visibility. To reduce the undulation effect, it is necessary to reduce the spacing between the various rods, but as explained above, this eliminates the possibility of varying the inclination of the rods and thus making adjustments in a wide range of radii of curvature of the mold formed by the rods.
When sheets of glass which are non-symmetrical in shape, i.e. trapezoidal in shape, or arranged asymmetrically with respect to the plane of longitudinal symmetry of the bending device are moved on the adjusted bent rods, a rotation of the sheets is produced which must be corrected in advance by arranging the sheets at an oblique angle to the direction in which they are moving before they contact the bent rods. Consequently, it is necessary to have ovens for reheating the glass which are larger than is strictly necessary, and the greater the difference between the length and the width of the sheets of glass, the greater is the size of the oven required. In some cases, it is impossible to obtain certain curvatures because of the size of the sheet.
There is another type of known bending device which provides glass of good optical quality, but it requires a very long operations time--a hundred or two hundred times as much as is required for the preceding devices--when a change in the radius of curvature of the glass sheet is desired. Such a device utilizes a shaper having a gaseous cushion formed by a multiplicity of blast plugs placed side by side. This device is well suited for bending of large amounts of sheets of glass having the same radius of curvature, but, on the other hand, it is not suited to producing short runs of sheets having different curvatures.